Year	per 1,000
1990	17.7
1991	17.1
1992	16.6
1993	16.1
1994	15.6
1995	15.2
1996	14.7
1997	14.2
1998	13.8
1999	13.4
2000	13
2001	12.7
2002	12.4
2003	12
2004	11.7
2005	11.3
2006	11
2007	10.7
2008	10.3
2009	10
2010	9.7
2011	9.4
2012	9
2013	8.7
2014	8.4
2015	8.2
2016	8
2017	7.9
2018	7.8
2019	7.7
2020	7.6
2021	7.5
2022	7.5